A power converter is a circuit having the dual functional task of changing a voltage level and providing a DC output voltage. This function entails the use of both a transformer to alter the voltage level from some input value and a rectifier to provide a DC output voltage. In instances of a step down converter where the level of the output voltage is reduced below the input voltage level, the output current exceeds the input current and hence the conductive losses on the secondary side of the transformer tend to dominate the overall dissipative loss of the entire converter. In fact the major portion of these conduction losses occur in the conductive path connecting the secondary winding of the transformer to the diodes of the rectifier. The fact that this connection carries a high AC current results in higher losses than that of a comparable DC current path. In addition to these dissipative losses the AC current path has stray inductance losses which further detracts from the desired performance of the converter.
Other factors in the performance of a converter are its density or space/footprint requirements on a circuit board and its thermal dissipation characteristics. The separateness of the transformer and rectifier require two distinct footprints and this requirement is spatially inefficient.